A method for releasing a parking brake, whereby a hydraulic fluid is fed into the hydraulic chamber in principle in the first step, is known from DE 10 2004 046 871 A1. The electric motor is then energized in such a way that it attempts to move the spindle nut of the brake device away from the brake disk. However, as a large spring force acts on the spindle via a spring element and a corresponding auxiliary piston, the electric motor does not at first succeed in moving the spindle. Only when the pressure in the hydraulic chamber is greater than the spring force of the spring element acting via the auxiliary piston is the spindle unloaded and begins to rotate. To prevent the spindle from jamming, a ball is provided on the head of the spindle in order to keep friction as low as possible. In addition, a stop comprising a first and a second element may also be provided to prevent jamming of the spindle, the first element being arranged on the head of the spindle and the second element on a housing part of the parking brake. Such design measures for preventing jamming of the spindle are, however, complex and costly as well as liable to failure.
A release process for a parking brake is further described in DE 10 2011 004 763 A1, whereby the electric brake motors of the two electromechanical brake devices, which together form the parking brake and are each installed on a respective wheel of a common axle, are first activated in the release device, so that the pressure of the brake pistons on the brake disk is reduced and the clamping force is decreased. As soon as a fault is detected during the release, both electric brake motors of the parking brake are switched off. After the electric brake motors have been switched off a maximum braking pressure is then built up automatically via the hydraulic vehicle brake and acts on the brake disks via the same brake pistons which are also subjected to pressure by the electric brake motors. A sufficient clamping force for securely locking the vehicle is thereby to be ensured. A recalibration is then carried out.
Automatic parking brake systems, or automatic “motor on caliper” parking brake systems, which are combined with electronic stability program (ESP) systems, are also known. These systems have at least one electromechanical actuator on the rear wheel brake, a spindle driven by a motor-transmission unit being located in the piston of the service brake. By rotation of the spindle a spindle nut guided in the brake piston is displaced, whereby the brake piston is also displaced. The clamping force on the rear axle can therefore be exerted either mechanically, hydraulically or in a combined manner. In commercially available systems a permanently energized DC motor is generally used to drive the electromechanical actuator.
However, in the event of a superposition of pressures through actuation of the service brake by the driver during actuation of the parking brake, it can happen that the spindle provided in the brake piston, especially when the hydraulic pressure applied by the driver is reduced again with the parking brake engaged, is exposed, on subsequent release without hydraulic pre-pressure, to high longitudinal forces acting via the spindle nut. These forces can cause the spindle 400 to flex elastically, as represented in exaggerated form in FIG. 1, since the spindle acts against the spring force F of the brake caliper (see arrow in FIG. 1: Fcaliper=ccaliper*spiston, where ccaliper is the spring constant of the brake caliper and spiston the distance travelled by the brake piston). Such flexing of the spindle can have the result that the frictional forces between the threads of the spindle and of the spindle nut increase during opening or release of the automatic parking brake (binding), whereby opening of the automatic parking brake without a corresponding pre-pressure applied by the driver may not be ensured.
It is therefore the object of the present disclosure to provide a method for releasing a brake device for an automatic parking brake, and a corresponding device or a control device, by means of which the parking brake can be released reliably and without stress on components.